Remove uses of the old plugin model from the analysis server

pkg/analysis_server/lib/src/provisional/completion/dart/completion.dart

-// Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
-// for details. All rights reserved. Use of this source code is governed by a
-// BSD-style license that can be found in the LICENSE file.
-
-/**
- * Support for client code that extends the analysis server by adding new
- * Dart specific completion contributors.
- *
- * Plugins can register Dart specific completion contributor factories.
- * The registered contributor factories will be used to instantiate new
- * contributors to get completions any time a client issues
- * a 'completion.getSuggestions' request.
- *
- * If a plugin wants to add completions, it should implement
- * [DartCompletionContributorFactory] by including code
- * like the following in the plugin's registerExtensions method:
- *
- *     @override
- *     void registerExtensions(RegisterExtension registerExtension) {
- *       ...
- *       registerExtension(
- *           DART_COMPLETION_CONTRIBUTOR_EXTENSION_POINT_ID,
- *           () => new MyDartCompletionContributor());
- *       ...
- *     }
- */
-import 'package:analysis_server/src/provisional/completion/dart/completion_dart.dart';
-import 'package:analysis_server/src/provisional/completion/dart/completion_plugin.dart';
-import 'package:plugin/plugin.dart';
-
-/**
- * The identifier of the extension point that allows plugins to register code
- * completion contributors. The object used as an extension must be a
- * [DartCompletionContributorFactory].
- */
-final String DART_COMPLETION_CONTRIBUTOR_EXTENSION_POINT_ID = Plugin.join(
-    DartCompletionPlugin.UNIQUE_IDENTIFIER,
-    DartCompletionPlugin.CONTRIBUTOR_EXTENSION_POINT);